This invention relates generally to improved perforating methods and apparatus and, more specifically, to novel shaped charge devices for use in perforating operations.
It has become common practice in the oil and gas industry to perforate the well casing of an oil and gas well to bring such well into production. Shaped charges have long been used for this purpose.
Oil well perforating shaped charges are often required to work in very restrictive environments. The logistics of transporting such devices from the warehouse to the field, the desire to keep the gun and borehole damage to a minimum as well as numerous other safety considerations dictate that a minimum amount of high explosive (HE) be used and that such HE be used most efficiently. The space constraints within a borehole further require that significant jet stretching takes place in the shortest possible standoff distance. It is also desirable to have a minimum of slug, and to have a jet with a high tip velocity, high velocity gradient, high density and high mass. A higher mass in the jet enlarges the jet diameter which in turn produces a larger entry hole while higher jet velocities increase the depth of penetration.
All of these objectives can be met with only limited success by employing a conventional shaped charge, wherein a conical or hemispherical cavity in a mostly cylindrical body of HE is lined with a conical or hemispherical liner of copper or other suitable material. In such shaped charges, the HE is initiated at the end opposite the liner. Detonation waves originating at this initiation point travel toward the liner apex then proceed toward the liner base. As a consequence of the enormous pressure exerted by the detonation, the liner moves toward the liner axis which is also the axis of symmetry. In the conventional design, the liner material arrives on the axis segment-by-segment where it divides into two parts, the jet and the slug. Typical jet tip velocities range from 5-8 km/sec depending on the liner material, cone angle and the amount and type of HE.
While the jet velocities of conventional shaped charges are fairly high, these velocities cannot be increased much further because of an inefficient explosive geometry. The detonation waves within such conventional charges impact upon the liner at oblique angles; therefore, a significant portion of the explosive energy is reflected away from rather than transmitted to the liner. This limitation on the jet velocities results in a limitation on the depth of penetration, which is further limited by the use of copper as the liner material. Copper is a popular choice because of its high ductility and low cost; however, copper's low density limits the pressure exerted by the jet and thereby limits the penetration.
These and other disadvantages are overcome by the present invention which employs a high density, sufficiently ductile liner material geometrically arranged in an implosion configuration. Implosion devices are inherently more efficient than point initiated devices because the detonation waves impinge upon the liner surface simultaneously at normal angles. This simultaneous impingement accelerates the entire liner simultaneously toward the center in a radially convergent fashion. In contrast, the liner of the conventional shaped charge is accelerated in sections from the apex to the base. The present invention also provides means and methods for accomplishing such simultaneous impingement so that the liner receives the impulse from the detonation wave simultaneously over the entire liner surface.